The precise cellular changes responsible for corpus luteum regression are unknown. Although a number of studies have been conducted on the role of membrane proteins (e.g., binding proteins), little work has been done on the role of membrane lipids. Therefore, this proposal will focus on the physical and chemical changes that appear in membrane lipids as tissue function declines. X-ray diffraction will be used to assess the phase changes that occur in lipid molecules during spontaneous and PGF2-alpha-induced regression. Previous experiments in plant and animal tissues indicate that membrane lipid undergoes physical alterations during tissue involution. In healthy cells the lipid molecules exist in a liquid-crystalline (fluid) state; however, during cellular regression regions of gel phase lipid appear. Presence of gel phase lipid may lead to a loss in intracellular comparmentalization, loss in ionic and metabolic gradients as well as disruption in membrane protein. The purpose of this study will be to examine if plasma and cytoplasmic membranes of the bovine corpus luteum undergo similar changes during loss of cellular function (progesterone secretion). Based on these results, the appearance of gel phase lipid will be documented as to membrane type and onset in relation to progesterone decline. These findings will indicate if the loss in membrane fluidity is correlated with the loss in corpus luteum function. Additional studies will be conducted to examine luteal cell membrane changes in more detail. These will include: measurement of gonadotropin receptor capacity, X-ray diffraction of liposomes prepared from microsomes, analysis of lipid compositional changes, differential scanning calorimetry of various phospholipid-cholesterol preparations, and X-ray diffraction of dispersed luteal cells. Where appropriate, corpus luteum function will be assessed by measurement of progesterone output. The above experimnts will provide detailed information on the molecular changes appearing in the cellular membranes during luteal tissue regression. Altogether, we feel that they will improve our understanding of the exact cellular changes that may be responsible for corpus luteum regression.